1. Field of the Invention
The present invention relates to a piezoelectric ceramic and more particularly, to a piezoelectric ceramic used for a surface mounting type piezoelectric part required to have heat resistance.
2. Description of the Related Art
A piezoelectric ceramic comprising lead titanate zirconate (Pb(Ti.sub.x Zr.sub.1-k)O.sub.3) or the like as a main component is widely used conventionally as the piezoelectric ceramic for an electronic device such as a ceramic filter, etc., and attempts have been made to add small amounts of various additives for improving the piezoelectric characteristics thereof.
More particularly, a material for a ceramic filter having excellent properties such as a flat group delay time (GDT) and a small phase distortion is required to have a low mechanical quality coefficient Qm. Known examples of such a material include a material comprising lead titanate zirconate containing niobium oxide, antimony oxide, tantalum oxide or the like as an additive, and a material comprising lead titanate zirconate in which Pb atoms are partly displaced by a rare earth element such as Sc, Ba, Ca, La or the like.
Although such a low Qm value piezoelectric ceramic causes no problem when the electrodes formed at both ends thereof are short-circuited, the piezoelectric ceramic has the problem that even if it has a high Curie temperature, the electromechanical coupling factor K decreases as the temperature rises when the electrodes are open, thereby significantly shifting the resonance and antiresonance frequencies. Therefore, when the piezoelectric ceramic is used for a surface-mounting type filter element, there is the problem that filter characteristics significantly deteriorate as the ceramic is heated to a high temperature (above about 250.degree. C.) during reflow soldering.
A method for solving the above problem has been reported in which a manganese compound is thermally diffused from a surface of the piezoelectric ceramic having a low Qm value and a high Curie temperature so that a high concentration of manganese oxide is distributed in a crystal grain boundary layer, thereby decreasing the resistivity of the crystal grain boundary portion and improving the heat resistance of the ceramic (Japanese Patent Unexamined Publication Nos. 6-1655 to 6-1657).
However, this method has a problem with respect to the production process in that if the structure of the crystal grain boundary of the piezoelectric ceramic which was previously sintered is changed or if the Pb amount of the piezoelectric ceramic is changed due to evaporation during the production process or if the inside of a thermal diffusion furnace has a wide temperature distribution, variations in the characteristics are increased when the manganese compound is thermally diffused from the surface of the piezoelectric ceramic. It is thus difficult to perform stable thermal diffusion of a large amount of manganese compound, and it is impossible to sufficiently decrease the resistivity of the crystal grain boundary portion and improve the heat resistance.